Opto-acoustic imaging devices and methods

ABSTRACT

In one aspect, the invention relates to a probe. The probe includes a sheath, a flexible, bi-directionally rotatable, optical subsystem positioned within the sheath, the optical subsystem comprising a transmission fiber, the optical subsystem capable of transmitting and collecting light of a predetermined range of wavelengths along a first beam having a predetermined beam size. The probe also includes an ultrasound subsystem, the ultrasound subsystem positioned within the sheath and adapted to propagate energy of a predetermined range of frequencies along a second beam having a second predetermined beam size, wherein a portion of the first and second beams overlap a region during a scan.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/983,417, filed on Nov. 8, 2007, now U.S. Pat. No. 7,935,060,which claims priority to and the benefit of U.S. Provisional PatentApplication 60/857,573, filed on Nov. 8, 2006, the entire disclosures ofeach of which are herein incorporated by reference.

FIELD OF INVENTION

This invention relates to the field of optical imaging and morespecifically to the design of fiber-optic probes for optical coherencetomography (OCT) and other optical imaging technologies, such asultrasound.

BACKGROUND

In recent years, the underlying cause of sudden heart attacks (acutemyocardial infarctions or AMI) has been the subject of much attention.The older prevailing theory of gradual occlusion of the coronary arteryhas been superseded by a new theory based on extensive histopathologicevidence that AMI is the result of a rupture in the coronary arterywall, specifically a rupture of a “vulnerable plaque.” A vulnerableplaque, also known as Thin-Capped Fibro-Artheroma (TCFA), ischaracterized by a thin fibrous cap covering a lipid pool located underthe artery wall. Conventional x-ray based angiographic techniques can beused to detect narrowing of the artery. However, directly seeing thesurface of the artery wall is essential to detect TCFA. Accordingly, aneed therefore exists for a probe design that enables detecting andvisualizing subsurface biological tissues and lipid pools.

SUMMARY OF THE INVENTION

The invention relates to methods and apparatus for imaging biologicaltissues and other materials using optical and acoustic imagingtechniques. A combination of Optical Coherent Tomography (OCT), aninterferometric imaging technology, and Intravascular Ultrasound (IVUS),is ideally suited to subsurface visualization of biological tissue, suchas the artery wall, via small-diameter probes. The disclosed methods arebased on a combination of IVUS (Intravascular ultrasound) and OCT(Optical Coherence Tomography) techniques that advantageously overcomesthe weakness of each individual technique. In particular, thecombination of both IVUS and OCT allows for a robust probe with manyadvantages.

IVUS is a medium-resolution (˜100 um), medium-penetration (˜2 cm)imaging technique. In contrast, OCT is a high-resolution (5-20 um),shallow-penetration (˜1 mm) technique. Neither technique individuallycan detect the state of the arterial wall. For example, the capthickness in a potentially hazardous TCFA can range from ˜25 um to ˜100um. This range is within the measurement resolution of OCT, but beyondthe measurement resolution of IVUS. Conversely, deep lipid pools beneatha thin cap greatly increases the risk of an AMI. OCT cannot be used toreadily penetrate such deep lipid pools, but IVUS can readily be used tovisualize such pools.

It is an object of the present invention to describe devices and methodswhereby IVUS and OCT can be performed simultaneously. It is a furtherobject of the invention to describe OCT optical sensors and IVUSultrasound sensors that can be combined into the same catheter deliverysystem.

One advantage of the invention is the aligned nature of the OCT andultrasound sensors such that co-registration of the cross-sectionalimages obtained by the two sensors can be obtained with high precision.Previous descriptions of such combined catheters did not provide theco-registration levels needed. Co-registration is important becausecoronary morphology changes rapidly, often in less than a millimeter oflongitudinal distance.

It is another object of the invention to describe a sensor structurewherein two probe beams are orientated at substantially the same anglewith respect to the longitudinal axis of the catheter. Again, this is tofacilitate proper co-registration of the images. Differing launch anglesof the probe beams implies that the two images diverge each other withdepth. Computational correction of this divergence is complex and canlead to errors in image presentation.

It is another object of this invention to describe efficient methods ofproviding both optical and electrical energy to the rotating sensorassembly at the tip of the catheter. Using various torque wire andcoated fibers to acts as co-axial signal lines saves valuable spacewithin a catheter body.

It is a further object of the invention to describe mechanisms andconfigurations of the probe that will simultaneously reduce unwantedparasitic acoustical and optical back-reflections while still providingan aligned and otherwise functional probe assembly.

It is yet another object of the invention to describe efficient rotarymechanisms for coupling both electrical and optical energysimultaneously into the catheter.

It is another object of the invention to describe a combined probeutilizing capacitive micro-machined ultrasonic transducers (CMUT) tocreate a dual element probe such that both the ultrasound and opticalbeams focus on substantially the same tissue spot simultaneously.

In one aspect, the invention relates to a probe. The probe includes asheath, a flexible, bi-directionally rotatable, optical subsystempositioned within the sheath, the optical subsystem comprises atransmission fiber, the optical subsystem capable of transmitting andcollecting light of a predetermined range of wavelengths along a firstbeam having a predetermined beam size. The probe also includes anultrasound subsystem, the ultrasound subsystem positioned within thesheath and adapted to propagate energy of a predetermined range offrequencies along a second beam having a second predetermined beam size.In one embodiment, a portion of the first and second beams scan the sameregion at different points in time. Alternatively, the first beam can bedirected to scan a first band of a region that is substantially adjacentto a second band of the region, wherein the second beam scans the secondband.

In another aspect, the invention relates to a system for medicalexamination. The system includes a first image processing device and asecond image processing device. The system also includes a probe, inelectrical communication with the first and second image processingdevices. In turn, the probe includes a first sensor of an imaging systemfor optical coherence tomography having an optical fiber for directingand emitting light into an area adjacent to a catheter tip introducedinto an examination area and for directing reflected light from theilluminated examination area to the first image processing device; and asecond sensor of an intravascular ultrasound imaging system fortransmitting and receiving acoustic signals to a second image processingdevice as electrical signals. Further, the system also includes adisplay device for outputting of images processed by the first and thesecond image processing devices.

In yet another aspect, the invention relates to an imaging probe adaptedfor insertion in a lumen. The probe includes a sheath having a core andan endface, an optical subsystem having an optical focus, the opticalsubsystem positioned within the core; and an array of ultrasoundtransducers having an acoustic focus, the array disposed on a portion ofthe endface.

In still another aspect, the invention relates to a probe. The probeincludes a sheath, a first ultrasound subsystem, the first ultrasoundsubsystem positioned within the sheath and adapted to propagate energyalong a first vector, and a second ultrasound subsystem, the secondultrasound subsystem positioned within the sheath and adapted topropagate energy along a second vector, wherein the first and secondvectors are substantially parallel and opposite in direction.

In yet another aspect, the invention relates to a method of imaging atissue region. The method includes the steps of inserting a combinationultrasound and OCT imaging probe in a lumen, and performing ultrasoundimaging, and then performing optical coherence tomography imaging. Inone embodiment of this method a flush solution is applied during theoptical coherence tomography imaging. In another related method of thisaspect, the ultrasound imaging is performed simultaneously with theoptical coherence tomography imaging.

In still another aspect, the invention relates to a method of imaging atissue region, the method comprising the steps of inserting acombination ultrasound and OCT imaging probe in a lumen, performingultrasound imaging simultaneously with optical coherence tomographyimaging whereby a flush solution is applied during the imaging.

Additional aspects of the invention include methods of fabricatingprobes that include sensor arrays, wherein each sensor includes anultrasound transducer and a driver.

It should be understood that the terms “a,” “an,” and “the” mean “one ormore,” unless expressly specified otherwise.

The foregoing, and other features and advantages of the invention, aswell as the invention itself, will be more fully understood from thedescription, drawings, and claims which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. The drawingsassociated with the disclosure are addressed on an individual basiswithin the disclosure as they are introduced.

FIG. 1A depicts a cross-sectional view of a longitudinally alignedIVUS/OCT probe according to an illustrative embodiment of the invention;

FIG. 1B depicts a probe utilizing a metal coated fiber with a shieldtube according to an illustrative embodiment of the invention;

FIG. 1C depicts a probe utilizing the coils of the torque cable assemblyas conductors according to an illustrative embodiment of the invention;

FIG. 1D depicts a cross-sectional view of the probe embodiment depictedin FIG. 1C;

FIG. 1E depicts a probe that includes two transducers adapted foroperating at different frequencies according to an illustrativeembodiment of the invention;

FIGS. 2 depicts a rotating coupling mechanism for delivering both RF andoptical energy to a rotating probe assembly according to an illustrativeembodiment of the invention;

FIG. 3 depicts a rotating coupling mechanism wherein the stationary coilis part of the probe interface unit according to an illustrativeembodiment of the invention;

FIG. 4 depicts a probe tip wherein CMUT technology is employed toachieve a dual focused beam according to an illustrative embodiment ofthe invention;

FIG. 5A depicts a fused OCT-IVUS schematic according to an illustrativeembodiment of the invention; and

FIG. 5B depicts a fused OCT-IVUS image according to an illustrativeembodiment of the invention.

The claimed invention will be more completely understood through thefollowing detailed description, which should be read in conjunction withthe attached drawings. In this description, like numbers refer tosimilar elements within various embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description refers to the accompanying drawings thatillustrate certain embodiments of the present invention. Otherembodiments are possible and modifications may be made to theembodiments without departing from the spirit and scope of theinvention. Therefore, the following detailed description is not meant tolimit the present invention. Rather, the scope of the present inventionis defined by the appended claims.

It should be understood that the order of the steps of the methods ofthe invention is immaterial so long as the invention remains operable.Moreover, two or more steps may be conducted simultaneously or in adifferent order than recited herein unless otherwise specified.

FIG. 1A illustrates a portion of an imaging probe 10 a, using aconventional IVUS ultrasonic transducer 12, an optical transducer 14which includes an angled-tip optical lens assembly 16 attached to asingle mode fiber 18, a standard miniature RF cable 20 delivering powerto the IVUS ultrasonic transducer, and a torque cable 22 providing astable revolution rate to the assembly.

Torque cables are generally preferred in this dual probe catheter as theoptical fiber is known to have a very low torsional (rotational)stiffness. For example, a 1 cm length of standard telecomm fiber 125 μmin diameter with approximately 1 millionth of a N-m of applied torquewill twist one degree. Therefore, it is unrealistic to expect the fiberto be sufficiently torsionally rigid to drive the complete assembly.

In FIG. 1A, both the optical transducer 12 and the IVUS ultrasonictransducer 14 are angled to minimize unwanted parasitic reflections fromreaching the respective transducers, and to create an alignedcross-sectional “cut” through the tissue. As shown, the acoustic beam(ab) emanating from the transducer is parallel to optical beam (ob)emanating from the fiber. The direction of these two parallel beams isrotated by an angle α relative to the longitudinal axis of the probe. Asshown in the figure, a small amount of longitudinal displacement isacceptable.

As a first order approximation, this allowable displacement is theapproximate maximum beam width of the combined probe 10 a. In mostcases, this will be the width of the ultrasound beam, which typicallyhas a width of ˜100 to 300 um (the OCT beam width is typically 25 um).Keeping the longitudinal displacement below this longitudinaldisplacement limit ensures the beams remain overlapped. Furthermore,having the two beams at 180 degrees opposite to each other ensureseasier real-time or post-processing alignment of the two images for anoverlay display.

FIG. 1B depicts a probe 10 b for imaging whereby the overall diameter isreduced. Here, a metal coated fiber 24 is shown inside an insulated tube26. These two cylindrical surfaces (tube and coating), the dielectricconstant of the insulation, and the insulation thickness can beconfigured to form a simple coaxial transmission line for the RFsignals. Such RF signals may vary from 10 to 60 MHz depending on theIVUS ultrasonic transducer design.

FIG. 1C illustrates another probe embodiment 10 c with a differentconduction mechanism. Specifically, in the probe 10 c shown, the inner28 and outer 30 coils of a torque cable 22 form a coaxial transmissionline 32. An insulated spacer 34 is inserted between the inner and outercoils to prevent a short circuit condition. The embodiment shown in FIG.1C allows RF power to be transmitted using an integral torque wire. Inone embodiment, the transducer is coated with epoxy. In one embodiment,both the ultrasound transducer and the optical fiber rotate together,being driven by the same torque wire. The distal tip epoxy encases theoptical fiber, the ultrasound transducer and its associated supplywires. Hence, the epoxy is selected for suitable optical and acousticproperties, as well as the required electrical insulation. Variousepoxies and silicon compounds can be purchased and/or specificallytailored that meet these requirements.

FIG. 1D illustrates a cross-section of the embodiment of FIG. 1C. Thetwo wires connected to the transducer shown in FIGS. 1C and 1D are rigidand rotate with the transducer.

FIG. 1E illustrates another optical probe embodiment wherein two IVUSultrasonic transducers T₁, T₂ operating at different frequencies areintegrated in the device. The lower frequency transducer T₁ allows forultrasound with s deeper scanning range, but lower resolution.Conversely, the higher frequency T₂ transducer allows for ultrasoundwith increased resolution but less depth penetration. In one embodiment,one transducer operates at about 5 MHz and the other transducer operatesat about 60 MHz. By using transducers with differing frequency ranges,an optical probe gains the advantages of both transducers, and mitigatesdisadvantages of each transducer, respectively. This dual transducerprobe achieves the same overall goals as the combined OCT/IVUS catheterin the case where very high resolution (˜10 um, OCT) is not needed infavor of very high penetration (˜3-5 cm) offered by a lower frequencyultrasound transducer.

FIG. 2 depicts a probe embodiment 40 that incorporates a mechanism fortransmitting both RF energy and optical energy to the rotating assembly.Specifically, a transformer scheme is used wherein a first coil 42 isattached to the rotating assembly 44, and a second coil 46 is integratedwith the connector 48 of the optical probe. This configuration has theadvantage that both coils move with the assembly during a ‘pull-back’(longitudinal) scan operation. Such pullbacks are used in both OCT andIVUS scans. When coupled with a rotation, a spiral scan pattern iscreated inside the arterial lumen. However, this approach results in anincreased cost for a one-time-use catheter.

FIG. 3 illustrates an alternative coupling scheme wherein the fixed coil42 is part of the drive unit 50 (motorized assembly providing rotationaland longitudinal motions). In this embodiment, the fixed coil ispermanent, and must be long enough to efficiently couple the RF energyinto the rotating catheter coil over the entire pullback length.Although incorporating the fixed coil to the drive unit imposesadditional requirements to the drive electronics, the decrease incatheter usage provides an overall cost savings.

Currently, conventional slip-ring technology is widely used in the fieldof optical imaging. Alternatively to FIGS. 2 and 3, slip-ring technologycan be used in IVUS probes described herein. However, for a probe with acentered optics configuration, the slip-ring is more difficult tomanufacture than in the IVUS-only case.

FIG. 4 illustrates an embodiment that includes capacitive micro-machinedultrasonic transducers (CMUT) 52 integrated in a coronary imaging probe54. The advantage of the CMUT is the small size of the transducer, whichis fabricated via conventional electronic CMOS processes. The small sizeand photolithographic fabrication allows customized arrays oftransducers to be built with the drive electronics on the samesubstrate. In this example, an array is formed in an annular regionaround the optical transducer. As a result, a co-focused, aligned andcombined beam can be formed, which eliminates the need for softwareregistration and removes a potential source of error. However, thisprobe tip may be larger than the embodiment shown in FIG. 1.

FIG. 5A illustrates a fused OCT-IVUS image 56, wherein the demarcationline 58 is chosen near the OCT penetration limit. As shown, byregistering the relative images of the ultrasound 60 and the OCT scans62, it is possible for a clinician to view a composite image that showsadditional physiological data. This approach can be used to imagesubsurface lipid pools.

FIG. 5B illustrates a fused OCT/IVUS image wherein the OCT portionappears in the image center and the IVUS portion appears in theperiphery. The outer boundary indicates approximately the boundary wherethe two regions intersect.

Not shown in the embodiments depicted in the figures is a guidecatheter. Typically, the guide catheter is a larger bore catheter usedto introduce the smaller imaging catheter into the main arterial trunk.From the guide catheter, a flush solution can be expelled to create aclear, blood-free imaging region when OCT imaging is performed.Alternative embodiments may include a flush lumen within the imagingcatheter whereby the flush solution is ejected at the imaging tip ratherthan from the guide catheter.

The aspects and embodiments of the invention can incorporate variouscomponents of varying dimension and materials as is known to one ofordinary skill. Various specific dimensions and materials are describedherein, however, these exemplary materials are not meant to be limiting,but only to evidence additional more specific embodiments. For all ofthe measurements discussed below, the dimension given also includes arange of greater than about 10-20% of the dimension given and less thanabout 10%-20% of the dimension given. In addition, for all of themeasurements discussed below, the dimension given also includes a rangeof greater than about 20-50% of the dimension given and less than about20%-50% of the dimension given. Further, in addition, for all of themeasurements discussed below, the dimension given also includes a rangeof greater than about 50-100% of the dimension given and less than about50%-100% of the dimension given.

In one probe embodiment, the viewing window used is a transparentepoxy-based window. Further, in another embodiment, the transducers usedhave a first dimension of about 0.1 mm and a second dimension of about0.5 mm. The forward viewing angle is about 10 degrees in one embodimentof the probe. The end-cap used in one probe embodiment includes a metal.The probe can include a hollow core that is substantially filled with anepoxy material in some embodiments. In one embodiment, the width of theshield RF cable is about 0.18 mm.

It should be appreciated that various aspects of the claimed inventionare directed to subsets and substeps of the techniques disclosed herein.Further, the terms and expressions employed herein are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed. Accordingly, what is desired to be secured by LettersPatent is the invention as defined and differentiated in the followingclaims, including all equivalents.

What is claimed is:
 1. An imaging probe having a longitudinal axis, the imaging probe adapted for insertion in a lumen having a lumen wall comprising: a probe tip defining a core and an endface, wherein a first end of the core terminates at the endface; a rotatable optical subsystem having an optical focus, the optical subsystem positioned within the core, the optical subsystem comprising an optical fiber having a fiber endface; and an array of ultrasound transducers having an acoustic focus, the array disposed on a portion of the endface, wherein the portion of the endface and the fiber endface are angled relative to the longitudinal axis such that the fiber endface receives scattered light from the lumen wall when the rotatable optical subsystem rotates.
 2. The probe of claim 1, wherein the array of ultrasound transducers is positioned concentrically around the optical subsystem and wherein the portion of the endface is an annular region.
 3. The probe of claim 1, wherein the acoustic focus and the optical focus are coincident.
 4. The probe of claim 1, wherein at least one transducer is a capacitive micro-machined ultrasonic transducer.
 5. The probe of claim 1, wherein the optical subsystem is configured to transmit and collect light of a predetermined range of wavelengths along a first beam having a predetermined beam size.
 6. The probe of claim 1, wherein the optical subsystem further comprises a lens.
 7. The probe of claim 1, wherein the lumen is a coronary artery.
 8. The probe of claim 1, further comprising a torque wire defining a bore, wherein the optical fiber is disposed in the bore.
 9. The probe of claim 1, wherein the optical subsystem is configured to transmit the scattered light to an optical coherence tomography system.
 10. The probe of claim 1 further comprising a sheath, wherein the probe tip is disposed in the sheath.
 11. An imaging probe adapted for insertion in a lumen having a lumen wall comprising: a probe tip defining a core and having an annular endface; a rotatable optical imaging system comprising an optical fiber having a fiber endface, the optical imaging system having an optical focus, the optical imaging system positioned within the core; and an intravascular ultrasound imaging system comprising an array of ultrasound transducers having an acoustic focus, the array of ultrasound transducers disposed on the annular endface, the annular endface and the fiber endface disposed at an angle relative to a longitudinal axis of the core.
 12. The probe of claim 11, wherein the array of ultrasound transducers is positioned concentrically around the optical imaging system and wherein the fiber endface receives scattered light from the lumen wall and the annular endface receives scattered acoustic waves from the lumen wall during imaging.
 13. The probe of claim 11, wherein the acoustic focus and the optical focus are coincident.
 14. The probe of claim 11, wherein at least one transducer is a capacitive micro-machined ultrasonic transducer.
 15. The probe of claim 11, wherein the lumen is a coronary artery.
 16. The probe of claim 11, further comprising a torque wire defining a bore, wherein the optical fiber is disposed in the bore.
 17. The probe of claim 11 further comprising a sheath, wherein the probe tip is disposed in the sheath. 